Coherent dynamically controllable narrow band light sources have many applications in medicine, life sciences, spectroscopy and environmental sensing. In many applications, it is preferable if the light source is controllable such that time-dependent intensity patterns, such as pulse patterns can be arbitrarily generated under electronic control. Further, it is often necessary to obtain high output power and in particular, high peak power in case of a pulsed operation. Finally, in many cases it is preferable if the light source can emit light of different wavelengths to make the light source more versatile.
A laser source which meets many of the above requirements is known from US 2006/0198397. This light source comprises a pulsed cascaded Raman laser which includes a pulse light source for generating a pulsed light having an optical spectrum centered at a source wavelength. A non-linear Raman conversion fiber is coupled to the pulse light source. The pulsed light traverses the non-linear Raman conversion fiber and the source power at the source wavelength is converted to an output power of an output signal having an output wavelength longer than the source wavelength due to a cascaded stimulated Raman scattering process.
In this prior art light source, a wavelength conversion is hence achieved by a cascaded stimulated Raman scattering. For this, the pulsed light signal is, after suitable amplification, combined with a Raman seed and introduced in a Raman conversion fiber. The seed-signal has a longer wavelength than the source signal and causes a first Raman scattering due to stimulated Raman emission in the Raman conversion fiber. Depending on the length of the Raman conversion fiber, further cascaded Raman scattering occurs, with correspondingly increasing the wavelength, such that different output wavelengths can be generated.
However, this prior art pulsed cascaded Raman laser also has drawbacks. In particular, one of the drawbacks is that the output bandwidth of the light source is generally limited by the bandwidth of the Stokes band of the Raman-active material. Namely, even if a Raman-active medium is pumped with a narrow band laser, the Raman scattered light generally has a certain bandwidth that reflects the width of the Stokes band of the Raman-active medium. With simulated Raman emission, a narrow band scattered light can be obtained if the probe light is of narrow width. However, in a cascaded Raman laser as described in the above prior art, stimulated Raman emission serves as the pump light for further Raman scattering, and this further Raman scattering is no longer stimulated but exhibits the emission bandwidth reflecting the entire Stokes band. For example, if an optical fiber is used as the Raman-active medium, a typical frequency shift in a certain range of 13 to 15 THz is obtained. Further, in the above-mentioned prior art, the light source lacks of flexibility in the time control of the light source.